Texas Instruments | How to Support 1.8-V Signals Using a 3.3-V LVDS Driver/Receiver Level-Shifter | Application notes | Texas Instruments How to Support 1.8-V Signals Using a 3.3-V LVDS Driver/Receiver Level-Shifter Application notes

Texas Instruments How to Support 1.8-V Signals Using a 3.3-V LVDS Driver/Receiver   Level-Shifter Application notes
How to Support 1.8-V Signals Using a 3.3-V LVDS
Driver/Receiver + Level-Shifter
Yaser Ibrahim, High-Speed Data and Clocks Group
This document discusses how to use a 3.3-V LVDS
driver or receiver with a level-shifter to support 1.8-V
(and other low-voltage) LVTTL/LVCMOS signals.
Introduction
An LVDS driver, such as the DS90LV011A, accepts a
single-ended LVTTL/LVCMOS input and translates it
to a differential LVDS output, as shown in Figure 1. An
LVDS receiver such as the DS90LV012A, on the other
hand, accepts a differential LVDS input and translates
it to a single-ended LVTTL/LVCMOS output.
VCC
VCC
Solution
Figure 3 shows an approach to overcome this problem
for the case when converting from low voltage singleended signal to LVDS. A level-shifter, such as
SN74AXC1T45, has 2 supply voltages, one for each
side (VCCA and VCCB). VCCB is connected to the same
voltage supply for the source of the input single-ended
LVTTL/LVCMOS signal, and VCCA is connected to the
3.3-V supply of the LVDS driver. The single-ended
input signal is fed to the level-shifter, which translates
it to 3.3 V, and the output of the level-shifter is fed to
the LVDS driver, which in turn converts it to LVDS
signal.
VDD(1.2V, 1.8V, 2.5V,«)
Single-Ended
LVTTL/LVCMOS
LVDS
Driver
LVDS
LVDS
Receiver
Single-Ended
LVTTL/LVCMOS
3.3V
VCCB
VCCA
Processor/
FPGA/ASIC
Single-Ended
Level
(1.2V, 1.8V, 2.5V, «) Shifter
Single-Ended
3.3V
LVDS Driver
LVDS
Figure 1. Operation of LVDS Drivers and Receivers
Sometimes there is a need to connect an LVDS driver
or receiver to devices that are powered by low
voltages (like 1.2 V, 1.8 V, 2.5 V, and so forth). The
single-ended outputs of such devices normally have a
voltage swing (VOH-VOL) that follows the supply voltage
(see Figure 2). Also, the single-ended inputs of such
devices accept a voltage swing (VIH-VIL) that follows
the supply voltage. Therefore, the LVDS driver or
receiver needs to support single-ended signals with
low voltage swings, but most LVDS drivers and
receivers available on the market have 3.3-V supplies
and only support 3.3-V LVTTL/LVCMOS signals.
VCC
VDD(1.2V, 1.8V, 2.5V,«)
Figure 3. Using a Level-Shifter to Interface a LowVoltage Device to a 3.3-V LVDS Driver
Figure 4 shows the implementation of this approach
when converting from LVDS to a low voltage, singleended signal. In this case, VCCA of the level-shifter is
connected to the same supply of the source of the
input single-ended LVTTL/LVCMOS signal, and VCCB is
connected to the 3.3-V supply of the LVDS receiver.
The LVDS input signal is fed to the LVDS receiver,
which converts it to a 3.3-V, single-ended signal for the
level-shifter. The level-shifter then translates the signal
to a low voltage, single-ended LVTTL/LVCMOS signal.
VDD(1.2V, 1.8V, 2.5V,«)
Single-Ended
(1.2V, 1.8V, 2.5V, «)
LVDS
Driver
LVDS
VCCB
VCCA
VCC
Processor/FPGA/ASIC
Processor/
FPGA/ASIC
Single-Ended
(1.2V, 1.8V, 2.5V, «)
3.3V
LVDS
Receiver
Single-Ended
(1.2V, 1.8V, 2.5V, «)
Level Single-Ended
Shifter
3.3V
LVDS
Receiver
LVDS
LVDS
Figure 2. The Need for Low-Voltage LVDS Driver
and Receiver
Figure 4. Using a Level-Shifter to Interface a 3.3-V
LVDS Receiver to a Low-Voltage Device
SPACER
SNLA307 – December 2018
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How to Support 1.8-V Signals Using a 3.3-V LVDS Driver/Receiver + LevelShifter Yaser Ibrahim, High-Speed Data and Clocks Group
Copyright © 2018, Texas Instruments Incorporated
1
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Multi-channel signals (2, 4, or 8) can be supported
when using this approach. Multi-channel LVDS drivers
are available, such as the DS90LV027A (dual),
DS90LV047A (quad), and SN65LVDS389 (Octal)
devices. Multi-channel LVDS receivers are also
available, such as the DS90LV028A (dual),
DS90LV048A (quad), and SN65LVDS388A (Octal)
devices. The level-shifters also have multi-channel
devices, such as the SN74AXCxx45 family.
Solution Cost
The additional components necessary for this solution
are:
1. Level-shifter chip
2. Two decoupling capacitors (one for each
supply)
The additional cost is low, but depends on the
selection of the level-shifter and discrete components
associated with it (the two decoupling capacitors). The
additional PCB area required for this solution is very
small, but again is dependent on the size of the levelshifter used. The SN74AXC1T45, for example, is
available in a variety of packages, including the spacesaving X2SON package with an area of less than 1
mm2.
2
Limitations
Some of the limitations for this approach are:
• The input signal voltage level supported is only
limited by the type of level-shifter used. The
SN74AXCxx45 family of devices support input
voltages from 0.65 V to 3.6 V, so single-ended
signals within this voltage range can be
supported.
• The supported data rate with this scheme is the
same as the supported data rate of the LVDS
driver or receiver and the level-shifter. The
LVDS drivers and receivers offered by TI
generally support data rates of 400 Mbps or
higher. The SN74AXCxx45 family of levelshifters support up to 500 Mbps when
translating from 1.8 V to 3.3 V.
Conclusion
Using a level-shifter with a 3.3-V LVDS driver or
receiver is a viable and economical option to support
lower voltage LVTTL/LVCMOS input signals such as
1.2 V, 1.8 V, 2.5 V, and more.
How to Support 1.8-V Signals Using a 3.3-V LVDS Driver/Receiver + LevelShifter Yaser Ibrahim, High-Speed Data and Clocks Group
Copyright © 2018, Texas Instruments Incorporated
SNLA307 – December 2018
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